1. Field of the Invention
This invention relates to improved surface passivation layers on GaAs crystal wafers, and more particularly to a post oxidation step which increases the compositional, physical and electrical uniformity of native surface passivation layers formed on GaAs crystal wafers by the As.sub.2 O.sub.3 vapor technique.
The generation of a doping impervious layer on the surface of a GaAs crystal wafer is a key to the advantageous use of these crystals in integrated circuit devices, in order that diffusion may be precisely controlled.
After a number of efforts to imitate the oxide masking technique used in silicon devices in GaAs devices, Takagi et al in J. Electrochem. Soc.: SOLID STATE SCIENCE AND TECHNOLOGY, Vol. 125, No. 4, April 1974, at pages 579-581, revealed that the use of a controlled vapor pressure of As.sub.2 O.sub.3 enabled growth of native oxide films on GaAs crystal wafers.
2. Description of the Prior Art
The above-mentioned Takagi et al article is the best prior art known to applicants. In the method there outlined, GaAs wafers and As.sub.2 O.sub.3 powder are heated in a closed evacuated quartz ampoule (pressure ca. 10.sup.-6 Torr.). In the presence of As.sub.2 O.sub.3 vapor generated by heating the ampoule at temperatures in excess of 450.degree., a thermal oxide is grown on the GaAs. The ampoule is cooled and water quenched. The oxides formed by the Takagi et al technique were not found to be reliably suitable for device application, in that disuniform surface layers were encountered, marked by variations in grain size, bubbles, localized concentrations, and other compositional and physical variations which were readily visible upon magnification. Electrical properties were noted to vary irregularly, too, in keeping with the nonuniform nature of the surface layer.